In UMTS (Universal Mobile Telecommunications Service) data processing, data pertaining to different propagation paths reach a mobile station at different instants of time. Moreover, said data may reach the mobile station from the same or different transmitting sources, i.e. base stations. The information transmitted in each path in the air interface is sampled, demodulated and delayed at the receiver, and then combined with the correspondingly processed information of the other paths to improve the quality of the received signals.
Typically, for code-division multiple access (CDMA) systems, spreading is used to translate each symbol into a sequence of chips. At the transmitter end, each symbol is multiplied by a wideband spreading code.
A CDMA wireless communication system, such as the 3GPP UMTS standard, comprises several base stations and several pieces of mobile user equipment (UE). Downlink data destined for the pieces of user equipment located in a particular base station's coverage area (cell) is spread in frequency prior to transmission using a direct spreading sequence code (called “spreading code” in the UMTS standard).
Due to the fact that a receiver is mobile, multiple propagation paths normally taken by the signal have different lengths and different attenuation resulting in a superposition of multiple, delayed, attenuated versions of the transmitted signal reaching the user equipment antenna. When the user equipment is in motion relative to a base station, the length of the propagation paths from base station to user equipment will change with time, so the corresponding delays will vary. Typically, a set of digital delay lines are used to realign the delayed versions of the signal before recombining them. The delay line may be envisaged as a shift register of length L, with a read pointer corresponding to a tap in the delay line. Arriving symbols are written to the start of the delay line, and in each symbol period all symbols are shifted one place along it. The content of the delay line at the tap is read every symbol period, resulting in a delayed version of the input symbol stream. The amount of delay is determined by the position of the tap, which should be programmable.
A more efficient implementation uses a single delay line before despreader units. This delay line has several taps, each of which is positioned corresponding to a different path delay. The output of each tap is a delayed version of the unprocessed, sampled input signal. If the tap outputs are each fed to a despreader unit, and the same scrambling code sequence alignment is used in each unit, the output of the despreaders will be a set of aligned symbol streams, which can be recombined without further ado.
The length of the delay line determines the maximum delay spread that can be accommodated by the receiver. The delay spread is defined as the maximum difference in the delay of the shortest propagation path and the longest propagation path, in the set of all possible paths (i.e. paths whose signal-to-noise ratio is sufficient for an acceptably low error rate).
To minimize cost, a delay line length will be set to the minimum required to accommodate the delay spread and the difference between base station timings.
The position of the taps in the delay line must be adjusted continually to realign the symbol streams.
As a result, it is likely that after a short period of time, one or more taps may reach the end or the start of the delay line. If the delay continues to change, it will no longer be possible to track the path, which will be lost.
The aim of the invention is to provide a mechanism for adjusting the position of the delay line with respect to the path timing, which avoids the loss of a path even with a reduced length for the delay line. The function of this mechanism is to move all the taps towards either the beginning or the end of the delay line, without losing the alignment of the symbols, nor the scrambling code sequences used for despreading.